1. Field of the Invention
The invention relates to an image transmission device for examining inaccessible parts of an object, for example channel-like parts of an object or other hollow spaces, having at least one structural group for conducting image information, and further having at least one focusable member as well as a focusing device coordinated with it.
2. Description of the Prior Art
A structural group for conducting image information via optical lenses arranged in series is disclosed by U.S. Pat. No. 3,608,998 (corresponding to German Pat. No. 1,566,112).
U.S. Pat. No. 3,010,357 (corresponding to German Gebrauchmuster 1,853,607) and German Pat. No. 1,269,287 disclose forming a structural group from glass fibers or other optically conducting fibers. These publications further disclose providing at least one focusable member within the structural group for focusing the group on the part of the object to be examined.
However, it is a disadvantage of such mechanical focusing devices that in spite of apparently sharp images in the viewfinder, photographic images made through such devices yield unsharp images. This occurs because during the visual observation of the part of the object, the image appears to be sharp to the observer at all distances, due to the automatic accomodation of his eyes.
Another disadvantage is that the distance of a part of an object observed or photographed cannot be determined accurately with known devices. This, however, is necessary when similar parts of objects are to be reproduced for comparative purposes on the same scale.
A device described in German Pat. Auslegensschrift No. 1,766,853 produces an electric signal as a function of the distance between the object and the device, by way of a photoelectric element, based on the light intensity reflected by the object. The signal is conducted to a calibrated recording or display device so that distance can be measured. To produce the reflection, light from a source of light with spectral characteristics such that the light emitted by it and reflected from the object will not affect the film intended for the generation of photographic images, is required. This technique interferes with the usefulness of the device because it renders the device subject to special preconditions.
Devices for the determination of distances are also known from German Pat. No. 2,156,617 and German Pat. No. 2,330,940 (corresponding to allowed U.S. application Ser. No. 673,726 and U.S. Pat. No. 3,781,110 respectively). With these devices, the location of at least one image plane of an imaging system can be determined by the principle of optical correlation. However, when the image plane determination technique described in these publications is to be used in combination with optical conductors having a grid structure over their cross section, a problem arises in that this grid structure hinders or renders impossible the filtering of the spatial frequencies present in the object. Specifically, this results in the phase relationship of the signals generated being lost during image transmission by means of such an optical conductor.